The diameter required for silicon single crystals has been tending to increase year after year. Presently, silicon single crystals of 6 inches in diameter are used for the latest device. Then, it is said that silicon single crystals of 10 inches or more in diameter will be needed in the future.
The silicon single crystals used in the field of LSIs are usually manufactured by the Czochralski method (CZ method) in which after a seed crystal has been dipped and adapted to the molten silicon contained within a rotating quartz crucible, the seed is slowly rotated and pulled, and according to this CZ method the molten silicon is decreased as a silicon single crystal grows. As a result, as the silicon single crystal grows, the dopant concentration in the silicon single crystal is increased and the oxygen concentration is decreased. In other words, the properties of the silicon single crystal are varied in the direction of its growth. Paralleling the tendency toward increasing the level of integration for LSIs, the quality required for silicon single crystals becomes increasingly severe year after year and this problem must be overcome.
As a means of overcoming this problem, a method has been known from old in which the interior of a quartz crucible according to the CZ method is divided by a cylindrical quartz partition member formed with small holes for passing molten silicon therethrough and a cylindrical silicon single crystal is grown on the inner side (a single crystal growing section) of the partition member while feeding starting material silicon to the outer side (a material melting section) of the partition member (e.g., Patent Publication No. 40-10184, "Detailed Explanation of the Invention", line 13 to line 28). The problem with this method is that solidification of the molten silicon tends to occur starting at the partition member on the inner side of the partition member as pointed out by Laid-Open Patent No. 62-241889 (page 2, "problems that the Invention is to solve", line 12 to line 16). In other words, the solidification is caused at the portion where the molten silicon surface of the crystal growing section is in contact with the partition member. This solidification grows towards the central portion of the crucible the temperature is low and the growth of the silicon single crystal is impeded. This is caused by the fact that since transparent silica glass usually used for the partition member tends to pass heat radiation and moreover in the usual case there is a considerable dissipation of heat to the water-cooled furnace wall from the portion of the top of the partition member which is exposed on the molten silicon surface, the heat in the molten silicon is transmitted upwardly through the partition member and the heat is dissipated from the portion of the partition member which is exposed on the molten silicon surface. As a result, the molten silicon temperature is greatly decreased in the vicinity of the partition member. Moreover, due to the intense agitation of the molten silicon, the surface temperature of the molten silicon attains a temperature which is uniform and slightly above the solidifying point. Thus, the molten silicon surface contacting with this partition member is in a condition having an extremely high tendency to cause its solidification. The previously mentioned Laid-Open Patent No. 62-241889 proposes a method employing no partition member in order to overcome this problem. In this method, however, the material melting section is so limited that particularly when a large-diameter silicon crystal is to be manufactured, it is difficult to melt the starting material silicon in an amount corresponding to the pulled amount of the silicon single crystal.
Recently, the production of high-quality granular silicon has been made possible and it is considered that to continuously feed such granular silicon as the starting material silicon into the molten silicon is relatively easy. However, if a sufficient heat for melting the granular silicon is not applied to the granular silicon when it is supplied onto the molten silicon surface, there is the possibility of causing a part of the granular silicon to remain unmelted. It is not infrequent that solidification is caused from the remaining unmelted granular silicon and the solidification spreads out. The reason is that due to the difference in specific gravity between the molten silicon and the granular silicon, the solid granular silicon floats to the molten silicon surface and its heat tends to be deprived of due to the radiation rate of the solid silicon being greater than that of the molten silicon. In particular, if the granular silicon is deposited and aggregated on the partition member at the molten silicon surface in the material melting section, as in the case of the solidification in the crystal growing section, the heat is rapidly lost through the partition member thus tending to cause the occurrence of solidification and its spreading. This problem is essentially the same even if the starting material silicon is in any other form than the granular silicon. In regard to this problem, in accordance with the method shown in Laid-Open Patent No. 61-36197 a "heat insulating cover" is arranged above the material melting section to facilitate rapid melting of the granular silicon (Claim 6).
Proposed by Laid-Open Patent No. 1-153589 is a method in which a partition member is employed and also the occurrence of solidification at the partition member is prevented. This invention proposes to completely cover the partition member by a heat keeping cover. In accordance with this method, the dissipation of heat from the partition member is prevented and hence the occurrence of solidification is prevented. Also, the melting capacity for the starting material silicon to be fed is sufficient. However, it has been found out that this invention is still insufficient for stably performing the growth of silicon single crystals.
The present invention has been made in view of these circumstances and it is an object of the present invention to provide a silicon single crystal amanufacturing apparatus which stably realizes the growth of a silicon single crystals over a long period of time.